DESCRIPTION (Adapted from Applicant s Description): Osteoporosis is a major public health problem. This application focuses on the problem early in life. Since there are few known safe, effective methods for restoring lost bone to the osteoporotic skeleton, prevention of osteoporosis is crucial. If skeletal development can be maximized during growth, young people will begin adulthood with optimal bone quality and be less likely to develop osteoporosis in later years. Yet bone health of children has not been adequately addressed. This proposal will build on 4 previous studies of bone quality in children. Bone quality in this application is defined as bone mineral content (BMC), determined by densitometry and patellar apparent velocity of ultrasound (AVU). The aims of this experimental study are 1) to test the effect of a weight-bearing exercise program on increases in bone quality in pubertal females beginning at age 9; and 2) to compare the effect of a weight-bearing exercise program with the effect of a high-calcium diet and a weight-bearing exercise program on increases in bone quality in pubertal females beginning at age 9. The primary hypotheses to be tested are: 1) Pubertal females who have completed a 3-year weight bearing exercise program will have higher spine BMC and patellar AVU than pubertal females who have maintained their usual activity level, and 2) pubertal females who have completed a 3-year weight-bearing exercise program and a high calcium diet intervention of 1500 mg/day will have higher spine BMC and patellar AVU than pubertal females who have completed the exercise program but have maintained their usual calcium intake. A convenience sample of 114 nine yr. old girls will be randomly assigned to 1 of 3 groups: 1) a group who will be enrolled in a weight-bearing exercise program which meets 3 times/week and who will consume their usual diets; 2) a group who will be enrolled in a weight-bearing exercise program which meets 3 times/week and will be given high calcium foods to supply 1500 mg/day; or 3) a control group who will consume their usual diet and maintain their customary activity level. Measurements of spine BMC, patellar AVU, usual dietary intake, height, weight, Tanner Stage, physical activity, and medical and social history will be made on all subjects at baseline, every three months, and at the end of 3 years. Evidence of a positive effect of increased weight-bearing exercise and/or increased calcium intake from food sources on bone quality will be followed by another proposal to determine if the treatment cohorts can maintain their skeletal advantage into late adolescence. The overall long-term aim of the P.I.'s work is to test interventions designed to bring about conscious increases in calcium intake and physical activity in children to improve their bone health and develop lifelong bone healthy behaviors.